A gas turbine, such as an aircraft turbojet engine, typically has an annular array of stator vanes located immediately upstream of a rotor wheel so as to direct the gases onto the rotor blades affixed to the wheel. Such vanes usually have airfoil cross-section with a concave surface and a convex surface interconnected by leading edge and a trailing edge portions. In order to obtain the maximum efficiency from the turbine gases throughout the wide variation in engine operating parameters, it is necessary to vary the gas flow passing over the stator vanes.
It is known to vary the cross-sectional area of the gas flow path between adjacent cooled stator vanes at the narrowest point (designated as the throat neck) by aerodynamic means. However, although these means are generally achievable at relatively low cost, they lead to appreciable reductions in the performance and interfere with the cooling of the vanes.
It is also known to vary the gas flow by mechanical means. This may involve varying the pitch of the entire stator vane or a portion of the vane. The pitch variation, however, creates problems regarding leakage of the gas flow as well as regarding the means referred to achieve adequate sealing.
It is also known to control the gas flow area by incorporating a series of flaps between adjacent stator vanes, which flaps may be extended so as to reduce the height of the gas flow channel. These flaps, when in their extended positions, create large areas of turbulent gas flow which precludes the achievement of optimum operating efficiency.